This invention relates to continuous roll-to-roll printing of substrates for roll-based soft lithography and more particularly to a centrifugal casting method for making a cylindrical polymer stamp.
Existing stamp casting methods typically create a planar stamp. This known technique is done almost exclusively by patterning a silicon wafer, then forming a casting cavity from some combination of rigid plates and spacers. The literature has many examples of this planar technique, in which variations include casting the stamp against a foil backplane, which can be used to maintain transverse rigidity or for mounting to a magnetic roll [2-8]. The prior art planar casting method has several limitations. The uniformity of the stamp thickness is dependent on the gaps between the mold plates which can be extremely difficult, if not impossible, to maintain with submicron uniformity for larger stamps. The master surface is relegated to existing wafer platforms and is not scalable beyond wafer sizes. Wafers can be dices and tiles, but in this case discontinuities in the stamp pattern are created. There are also limitations on area and flatness. Wrapping a flat stamp around a roll presents a significant registration problem. Additionally, wrapping the flat stamp around a body of curvature results in strain at the printing interface. This strain causes pattern distortion that limits the thickness of the stamp that can be used, although ideally thicker stamps could be used to desensitize the contact interface [9]. Further, applying a flat stamp to a rigid roll results in an inherent seam. This results both in pattern discontinuity and stamp discontinuity; the latter can result in adverse dynamic performance under high speed rolling [8].
Centrifugal casting is another known prior art casting technique. In centrifugal casting, a fluid is deposited on the inner surface of a rotating drum and allowed to solidify or cure while the drum is rotating. The centripetal acceleration from the associated rotation forces the fluid into a uniform layer on the inner surface of the drum. Usually, centrifugal casting is performed with phase-change materials (i.e., molten iron for pipes or melted thermoplastics for bushings).
It is therefore an object of the present invention to provide a centrifugal casting method that eliminates pattern discontinuities (seams), produces a more dimensionally uniform stamp thickness, and can greatly reduce residual stresses and strains.